a) Field of the Invention
The invention is directed to a lifting device with four pairs of hoisting cables arranged in a V-shape relative to one another for lifting and lowering a load suspension apparatus at which they engage and which are taken off from drivable cable drums, the free lengths of the hoisting cables of two longitudinal pairs of cables lie in longitudinal planes which are located at a distance from one another, and the free lengths of the hoisting cables of two transverse pairs of cables lie in transverse planes which are located at a distance from one another and which extend at right angles to the longitudinal planes.
b) Description of the Related Art
A known lifting device of the type mentioned above is used in container cranes, whose purpose is to stack containers in several layers, to load and unload trucks, and so on. The cranes are controlled entirely in a fully automatic or semi-automatic manner. The container or the load suspension apparatus carrying the container must be positioned accurately within a range of centimeters. This calls for a load suspension which is as stable as possible and which ensures that no uncontrollable pendulum movements can occur due to the influence of external forces. A design that has proven itself in this respect comprises a cable shaft or cable tower having four pairs of cables, each pair being formed by cables arranged in a V-shape relative to one another. The free lengths of two longitudinal cable pairs lie in longitudinal planes located at a distance from one another, and the free lengths of two transverse cable pairs lie in transverse planes which are located at a distance from one another and extend at right angles to the longitudinal planes.
In the previously known lifting devices of the type mentioned above, two cable drums are disposed at right angles to one another, one cable drum being used to wind up and wind off the longitudinal cable pairs and the other to wind up and wind off the transverse cable pairs. One of the two cables of a cable pair runs from the cable drum directly to the load, and the other runs from the cable drum to the load over a deflection pulley to form the V-shaped arrangement of the two cables. The two cable drums are driven synchronously by a common driving motor via an associated transmission to raise and lower the load suspension apparatus in a desired manner.
In another previously known lifting device with a cable shaft or cable tower of the type described above, all of the cables of the cable pairs are wound on a central cable drum and run over or around corresponding cable rollers (deflection pulleys) to form the V-shaped cable pairs. The central cable drum is driven by a driving motor via a transmission for lifting and lowering the load suspension apparatus.
These previously known lifting devices are disadvantageous in that the adjustments of the lengths of the individual cables are very time-consuming. On the one hand, the adjustments for the cable lengths determine the geometry of the suspension of the load suspension apparatus, and this load suspension apparatus must be suspended centrically as far as possible. Further, the cable lengths must match each other as exactly as possible so that the cables are loaded as uniformly as possible, because different cable tensions shorten the lifetime of the cables. Changes in cable tensions come about over the course of operation so that the cable adjustments must be repeated occasionally. Also, the ratios of the cable tensions can change depending on the operating state so that an optimal adjustment for all operating states may no longer be possible at all.
The size of the different hoisting cable loads is determined particularly by the following influences: positional tolerances of the cable drums and cable pulleys; deformations of the carrying frame for the hoist and parts thereof and deformations of the load suspension apparatus; diameter tolerances and concentricity tolerances of the cable drums, diameter tolerances and concentricity tolerances of the cable pulleys around which the hoisting cables run; diameter tolerances of the cables; differences in the stretching behavior of the cables; different cable lengths and the consequent different elongation behavior; the accuracy of the cable adjustments.
Since uniform cable tensions are crucial in determining the lifetime of the hoisting cables, the greatest possible precision is required for the manufacture and placement of the structural component parts. The cables must come from the same batch so that the diameter and stretching behavior are as identical as possible. Maintenance costs are high, and the lifting device is not available during the time-consuming maintenance work. The expected lifetime of the cables is still relatively low even when these criteria are met.